With the known methods of chronic blood purification therapy such as hemodialysis, hemofiltration and hemodiafiltration, the patient's blood is passed through an extracorporeal system. Arteriovenous fistulas, vascular implants and/or various catheters are used as access to the patient's vascular system. Typical flow rates in a vascular access are in the range of 1100 mL/min. The patient is usually connected to the extracorporeal system by dialysis cannulas which are used to tap into the fistula or the vascular implant.
If the connection between the extracorporeal system and the vascular system becomes undone or a blood leak occurs in the extracorporeal system, the patient can be prevented from bleeding to death only if the extracorporeal blood flow is stopped within a few minutes. Therefore, extracorporeal blood systems are generally equipped with protective systems to constantly monitor the arterial and venous pressure (P.sub.art and P.sub.ven) within the system as well as the admission of air into the extracorporeal system.
In the event of an alarm, the blood treatment is stopped, the venous clamp is closed and acoustic and optical warning signals are triggered. The protective system based on the pressure measurement responds when the arterial or venous pressure in the extracorporeal system changes by more than .+-.60 mm Hg. The alarm limits are selected so that a change in the position of the patient does not trigger an alarm.
If the connection between the patient and machine becomes undone at the arterial connection, i.e., at the cannula establishing the patient's blood flow to the extracorporeal system, the pressure-based protective system on the machine responds rapidly, as explained below. The dialysis cannula presents the greatest flow resistance in the extracorporeal system. When air is drawn through the cannula into the arterial vacuum system of the extracorporeal system, the flow resistance of the cannula drops by a factor of 10.sup.3 in proportion to the difference in density between the blood and air. Thus, the arterial vacuum in the extracorporeal system collapses suddenly.
However, the response of the pressure-based protective system is not always guaranteed in the case when the venous cannula has become detached from the vascular access. On the venous side, the purified blood is supplied to the patient at an excess pressure, with the venous excess pressure being proportional to the delivery rate of the blood pump. This prevents penetration of air through the cannula into the extracorporeal system, which would be the case on the arterial vacuum side. Therefore, the flow resistance of the venous cannula does not change, and the venous pressure on the machine end drops only by the amount of the pressure in the patient's vascular access. Thus, the change in venous pressure in the extracorporeal system is usually too low to trigger the pressure-based protective system to respond. The additional hydrostatic pressure difference between the venous pressure sensor and the cannula triggers a machine alarm only when the venous cannula is definitely below the fistula after slipping out of the vascular access.
In the case of a blood leak in the venous tubing system, it may also occur that the resulting venous pressure drop is not sufficient to guarantee that the existing pressure-based protective system will be triggered.
In addition to the above method, where the pressure in the arterial branch of the extracorporeal system is monitored to detect whether the arterial cannula has slipped out, and where the pressure in the venous branch of the extracorporeal system is monitored independently of the pressure monitoring in the arterial branch to detect whether the venous cannula has slipped out, there are known monitoring systems that monitor pressure pulses propagating in the extracorporeal system.
International Patent No. WO 97/10013 describes a dialysis machine having such a monitoring system which monitors the pressure pulses in the venous blood line produced by the blood pump in the arterial blood line.